Boarding his shuttle, he ordered the pilot to lift off. A pity, my son, he thought. You could have joined me and together...we could have destroyed the Emperor and ruled the galaxy in his place. As he stared at the severed appendage in his hands, a sudden flash of insight struck the Dark Lord, realization dawning like the sunrise of Bespin. Perhaps, if you will not be turned, little Jedi, a suitable substitute may be arranged.
Suddenly, Vader was struck to his knees by the horribly powerful voice that rolled like fiery thunder through his brain. The pilots struggled vainly to ignore the Dark Lord's...discomfort. "Yes, my servant," the voice boomed in his mind, dripping raw evil. "Come to Mount Tantiss, immediately. I shall meet you there, and we will discuss my new trophy."
"Yes...my Master," Vader gasped, feeling an icy stab of dread in his soul, as the Emperor's mocking chuckle still echoed in his mind. His Master had detected his rebellious thoughts. This discussion would be most unpleasant. Most unpleasant indeed.
Source: Clone B-2332-54

Boarding his shuttle, he ordered the pilot to lift off. A pity, my son, he thought. You could have joined me and together... we could have destroyed the Emperor and ruled the galaxy in his place. As he stared at the severed appendage in his hands, a sudden flash of insight struck the Dark Lord, realization dawning like the sunrise of Bespin. Perhaps, if you will not be turned, little Jedi, a suitable substitute may be arranged.

Suddenly, Vader was struck to his knees by the horribly powerful voice that rolled like fiery thunder through his brain. The pilots struggled vainly to ignore the Dark Lord's... discomfort. "Yes, my servant," the voice boomed in his mind, dripping raw evil. "Come to Mount Tantiss, immediately. I shall meet you there, and we will discuss my new trophy."

"Yes...my Master," Vader gasped, feeling an icy stab of dread in his soul, as the Emperor's mocking chuckle still echoed in his mind. His Master had detected his rebellious thoughts. This discussion would be most unpleasant. Most unpleasant indeed.

Clone B-2332-54

There's also when he easily fogged Vader's senses from across the galaxy:

There's also this, where the relationship between Vader and the Emperor is stated to be akin to a priest and his god:

When the hologram of the Galactic Emperor finally spoke, it did so with a voice even deeper than Vader's. The Emperor's presence was awesome enough, but the sound of his voice sent a thrill of terror coursing through Vader's powerful frame. "You may rise, my servant," the Emperor commanded.

Immediately Vader straightened up. But he did not dare gaze into his master's face, and instead cast his eyes down at his own black boots.

"What is thy bidding, my master?" Vader asked with all the solemnity of a priest attending his god.

The Empire Strikes Back novelization

__________________"I created Azronger in a laboratory. He is Sheev's chosen vessel." - The_Tempest

"We used to have this problem in Clone Wars when we would try to put Yoda in a story and it would be like, “Yeah, if Yoda’s there, this isn’t really a problem is it?” That’s because Yoda’s going to go in there and kick everyone’s butt. We all felt that Ahsoka, the only person that could really match her in this time period, blow for blow, would be Vader or the Emperor."

I thought I recalled something else being added onto this quote. Something Filoni or someone said about the quote not really being a direct statement of how powerful she was, but rather, being more like Ahsoka being the only person at the time, save Yoda and Obi-Wan, that had the same kind of formal training as Vader or Sidious, which was the reason only Vader and the Emperor could match or beat her. Does anyone have that quote? I swear to God I've seen it before.